The term "superalloy" is used to represent complex nickel, iron, and cobalt based alloys containing additional metals such as chromium, aluminum, titanium, tungsten, and molybdenum. The term "based" as used herein means that that element is the largest weight fraction of the alloy. The additives are used for their high values of mechanical strength and creep resistance at elevated temperatures and improved oxidation and hot corrosion resistance. For nickel based superalloys, high hot strength is obtained partly by solid solution hardening using such elements as tungsten or molybdenum and partly by precipitation hardening. The precipitates are produced by adding aluminum and titanium to form the intermetallic compound .gamma.' ("gamma prime"), based on Ni.sub.3 (Ti,Al), within the host material.
The properties of superalloys make them desirable for use in corrosive and/or oxidizing environments where high strength is required at elevated temperatures. Superalloys are especially suitable for use as material for fabricating components such as blades, vanes, etc., for use in gas turbine engines. These engines usually operate in an environment of high temperature and/or high corrosiveness. Therefore a need exists for alloys with high temperature oxidation resistance and/or good hot corrosion resistance.
Nickel based superalloys are well known in this field. For instance, U.S. Pat. No. 4,261,742 to Coupland et al. discloses a superalloy consisting essentially of 5 to 25 wt % chromium, 2 to 7 wt % aluminum, 0.5 to 5 wt % titanium, at least one of the metals yttrium and scandium present in a total amount of 0.01 to 3 wt %, 3 to 15 wt % in total of one or more of the platinum group metals, and the balance nickel. The Coupland et al. superalloy has increased oxidation and hot-corrosion resistance and may be used as a material for fabricating blades or vanes of gas turbine engines or components used in coal gasification systems. Also, U.S. Pat. No. 4,018,569 to Chang discloses an alloy consisting essentially of 8 to 30 wt % aluminum, 0.1 to 10 wt % hafnium, 0.5 to 20 wt % of an element selected from the group consisting of platinum, rhodium and palladium, 0 to 3 wt % yttrium, 10 to 40 wt % chromium, and the balance comprising an element selected from the group consisting of iron, cobalt and nickel. The Chang superalloy has improved environmental resistance which may be used to improve the temperature capability of components in gas turbine engines. However, neither Coupland et al. nor Chang disclose superalloy compositions containing palladium in amounts sufficient to improve the weldability of the superalloy in accordance with the requirements of the present application. These patents are hereby incorporated by reference.
Other patents are known that disclose high temperature nickel containing alloys. Some examples include: U.S. Pat. No. 4,149,881 to D'Silva, U.S. Pat. No. 4,414,178 to Smith, Jr. et al., U.S. Pat. No. 4,719,081 to Mizuhara, and U.S. Pat. No. 4,746,379 to Rabinkin, all hereby incorporated by reference. These patents disclose alloys with various amounts of palladium, chromium and nickel but do not contain aluminum which is a required element of the present invention.
Current and next generation turbofan turbine engines use nickel based superalloys for many of the components in the high temperature sections of an engine. These sections include the later stages of the high pressure compressor, the combuster, the high and low pressure turbine, and the exhaust modules. These components are subjected to a wide variety of service related degradation including oxidation, fatigue, creep, corrosion, and erosion. In nearly all applications, more than one of these phenomena occurs during turbine engine operation. As a result, alloy design principally has been concerned with improving the thermomechanical properties of the alloys. Produceability of the alloy, i.e., weldability, castability, forgeability, and machineability are often considered a secondary or tertiary criterion during alloy design. However, when weldability is considered during alloy design the resulting material may be widely used. For example, Alloy 625 and its derivatives (including Alloy 718) are the most widely used superalloys in the world [H. L. Eiselstein and D. J. Tillack "The Invention and Definition of Alloy 625", Superalloys 718, 625 and Various Derivatives, Conference Proceedings, Pittsburgh Pa., June 1991, ed. E. A. Loria].
To improve the oxidation resistance and strength of Ni alloys, successive generations of alloys have incorporated increasingly higher levels of aluminum and to a lesser extent titanium. Both Al and Ti are detrimental to weldability.
There are several modes of cracking that can occur during welding. One of the most troublesome is strain age cracking of the weld metal or in the heat affected zone of the base material. Strain age cracking is the principal reason why nickel based superalloys are considered to be difficult to weld [Welding Handbook Vol. 4, Seventh Edition, ed. by W. H. Kearns, p. 233 and 236, .COPYRGT.1982 American Welding Society]. This type of cracking can occur during cooling from weld temperature, during post weld heat treatment, or during the application of subsequent weld passes. The primary reason these alloys exhibit strain age cracking is that the aging kinetics of the .gamma.' phase is very fast and the alloy can not accommodate the resulting strain without cracking. FIG. 1 shows the relationship between an alloy's Al+Ti content and weldability [M. Prager and C. S. Shira, Weld. Res. Counc. Bul., 128, 1968]. Note that alloys containing greater than about 3 wt % Al are considered difficult to weld, in addition as Ti levels increase the allowable amount of Al present in the alloy also decreases. Also note that this chart was developed before applicant's discovery of the affect of the addition of palladium to superalloys, which allows higher amounts of Al+Ti to be included in the composition at the same level of weldability. This is discussed more fully below.
For alloys that lie close to the line, such as Rene'41 and Waspaloy, special heat treatments have been used to reduce cracking. For example, over aging Rene'41 has been shown to reduce strain age cracking through the coarsening of the .gamma.' phase [W. P. Hughes and T. B. Berry, "A Study of the Strain-Age Cracking Characteristics in Welded Rene'41-Phase 1", Welding Journal, August 1967, p 361-370].
It is common for current generation superalloys to have as much as 12% Al with little or no Ti present. The impossibility of welding these alloys has a significant impact on the repairability of components made from such alloys. For example, a turbine blade may be removed from service due to tip wear while the component still has a significant portion of its design life remaining. It is desirable to weld repair the worn area and return the component to service. Currently these components are repaired using a solid solution strengthened alloy such as Alloy 625, Hastelloy X, L605, or HS188. However, these alloys lack the strength and oxidation resistance of the original material; as a result the repaired components suffer rapid degradation during subsequent service.
Several other types of cracking can occur in superalloy weldments. For castings and large grain wrought materials grain boundary liquation cracking or hot shortness may occur. This type of cracking is minimized by using a low heat input process such as laser, electron, or micro plasma arc welding and controlling the level of carbide forming and impurity elements [T. J. Kelley, "Welding Metallurgy of Investment Cast Nickel-Based Superalloys", Weldability of Materials, Conference Proceedings, ed. R. A. Patterson and K. W. Mahin, .COPYRGT.1990 ASM International]. Also, weldments can also suffer from nil ductility cracking and restraint cracking. Both of which are best minimized by proper weld schedule development and process control.
Current generation Ni based superalloys derive their oxidation resistance from the formation of an extremely adherent and cohesive Al.sub.2 O.sub.3 surface layer. The formation of the Al.sub.2 O.sub.3 film depends on the Al content of the alloy and other elements such as Cr, Y, Hf, and Ti [C. T. Sims and W. C. Hagel, eds., The Superalloys, .COPYRGT.1972 Wiley, N.Y.]. However, increasing aluminum content is the most effective method of improving oxidation resistance. Increasing the aluminum content is limited by the need to balance other thermomechanical properties. As a result oxidation resistant coatings have been developed to increase the Al content at the surface. One technique is to apply a diffusion aluminide coating where Al is applied by a pack cementation or a chemical vapor deposition process. Other coating systems are based on the MCrAlX (M can be Ni and/or Co and X can be Y and/or Hf) alloys. These alloys are similar to superalloys except they are very high in Al and contain as much as 1.5% Y or Hf. These coatings are applied by physical vapor deposition or a thermal spray process. One variation of the above coating is to electroplate onto the surface of a component Pd to improve the oxidation and corrosion resistance [S. Alperine, P. Steinmetz, A. Friant-Costantini, P. Josso, "Structure and High Temperature Performance of Various Palladium-Modified Alumined Coatings: A Low Cost Alternative to Platinum Aluminides," Surface and Coating Technology, 43/44 (1990), 347-358; P. Lamesle and P. Steinmetz, "Growth Mechanisms and Hot Corrosion Resistance of Palladium Modified Aluminide Coatings on Superalloys", Materials andManufacturing Processes, vol. 10, no. 5, 1053-1075, (1995)].
At Penn State, work has been performed studying the effects of Pd on the oxidation behavior of Mo--Cr and Mo--W--Cr alloys [D. Lee and G. Simkovich, "Oxidation of Molybdenum-Chromium-Palladium Alloys," Oxidation of Metals, 34, Nos. 1/2, (1990); D. Lee and G. Simkovich, "Oxidation of Mo--W--Cr--Pd Alloys," Journal of Less Common Metals, 163 (1990), 51-62]. The results show that 1-3 wt. percent Pd is sufficient to significantly improve the high temperature oxidation resistance of the alloy systems. The researchers hypothesized that Pd acts as a Cr reservoir for the formation of Cr.sub.2 O.sub.3 and as a barrier to the inward diffusion of oxygen. There have not been previous studies on the effects that Pd additions have on the oxidation resistance of Ni based superalloys.
Previous work on platinum additions to superalloys has shown a beneficial effect on oxidation behavior at high temperature. Platinum concentrations of about 1-3 weight percent were shown to significantly reduce the high temperature oxidation rate of the base metal. The improvement was attributed to an increase in the diffusion rate of other species [I. M. Allam, H. C. Akuezue, and D. P. Whittle, "Influence of Small Pt Additions on Al.sub.2 O.sub.3 Scale Adherence", Oxidation of Metals, Vol. 14, No. 6, 1980]. This may be due to an increase in lattice parameter of the .gamma. phase caused by the presence of Pt. In the presence of Hf, Pt promotes inwardly growing Al.sub.2 O.sub.3 pegs that reportedly increased scale adherence [G. J. Tatlock and T. J. Hurd, "Platinum and the Oxidation Behavior of a Nickel Based Superalloy",Oxidation of Metals, Vol. 22, Nos. 5/6, 1984]. It is possible that Pd additions may also increase oxide scale adherence by the same or other mechanisms.
The surface segregation of Cr, Pd, Mo, and Ni for a high chromium ferritic stainless steel has bee studied [W. E. Delport and J. P. Roux, "The Surface Segregation and Oxidation of Chromium and Palladium in High Chromium Stainless Steels", Corrosion Science, Vol. 26, No. 6, pp. 407-417, 1986]. The investigators found that at 550.degree. C. palladium oxidation is virtually complete before the oxidation of chromium begins. Also, the data suggests that Cr diffuses more rapidly through PdO than through the bulk material. This data suggests that the passivation characteristics of a ferritic stainless steel would be improved if a small amount of palladium (approximately 0.4 weight percentage) is added to the steel. Unfortunately, the study did not investigate high temperatures, where the formation of PdO can not occur.
Gas turbine engines are used in a wide variety of applications including commercial and military aircraft and for electrical power generation. Fuel efficiency is a major concern for turbine manufacturers and operators. Considerable effort is expended during the design of turbines to improve fuel efficiency over earlier models, and operators spend a large part of their maintenance effort to maintain fuel efficiency. Fuel represents a major cost for both airlines and electric utilities.
Fuel efficiency is increased over earlier engines by incorporating new designs that take advantage of advances in aerodynamics and computer simulation. Fuel efficiency is also increased by incorporating advanced materials that allow the engine to operate at higher combustion temperatures. Higher combustion temperature results in more complete burning of the fuel. New materials are usually more expensive due to an increase in raw material and manufacturing costs. Often these costs are more than offset by a decrease in fuel costs. Superalloys have been used extensively in the hot sections of turbine engines because of their high strength and excellent resistance to oxidation (usually with the addition of a coating). Unfortunately superalloys are very difficult to fusion weld. The inability to fusion weld superalloys results in increased new part manufacturing cost and an increase in maintenance costs. It is desirable to develop a new alloy that has both excellent oxidation resistance and is more weldable than current alloys.
Turbine efficiency is reduced when excessive clearances develop between rotating components and stator components. In the turbine, unwanted clearances develop due to the thermomechanical degradation of the blade tip allowing airflow to leak past the blades. Often turbine blade tip degradation becomes severe enough for the operator to remove the blade from service for repair. The repair consists of welding a sufficient amount of repair material to the tip and recontouring the blade to final dimensions. The repair material is often Alloy 625. This material is a solid solution strengthened nickel alloy that has inferior oxidation resistance to the original blade material. However, Alloy 625 exhibits excellent weldability compared to most original blade materials which have such poor weldability that they can not be used as the repair material.
Because most repair material, frequently Alloy 625, has poor oxidation resistance, it does not maintain clearances and causes the turbine blades to be removed frequently for additional repairs. By substituting Alloy 625, or another repair material, with the subject invention, the turbine operator will realize a reduction in fuel consumption and maintenance costs